MEASURING INSERT WITH PROTECTIVE TUBE

Description

[0001] The invention relates to a device for determining and / or monitoring at least one process variable of a medium. The medium is preferably located in a container, for example a tank or a pipeline. The device comprises a measuring insert with at least one sensor element, arranged in particular in an end region of the measuring insert, for detecting the process variable, and an immersion body for receiving the measuring insert, which immersion body projects at least temporarily and / or partially into the medium, and which immersion body surrounds the measuring insert at least in a partial region facing the medium, in which region the at least one sensor element is preferably located. The sensor element is, for example, a temperature sensor which serves to detect the process variable temperature.

[0002] Thermometers have become known in a wide variety of designs from the state of the art. There are thermometers that use the expansion of a liquid, gas, or solid with a known coefficient of expansion to measure temperature, and there are also those that relate the electrical conductivity of a material or a derived value to temperature, such as electrical resistance when using resistance elements or the thermoelectric effect in the case of thermocouples. In contrast, radiation thermometers, especially pyrometers, utilize the thermal radiation of a substance to determine its temperature. The underlying measurement principles have been described in numerous publications.

[0003] A temperature sensor in the form of a so-called thin-film sensor, particularly a resistance temperature detector (RTD), uses a sensor element provided with connecting wires and applied to a carrier substrate. The back of the carrier substrate is usually coated with metal. So-called resistance elements, such as platinum elements, are used as sensor elements and are also commercially available under the designations PT10, PT100, and PT1000, among others.

[0004] The resistance elements are often inserted into a sensor head, for example, a sensor tip, using a soldering process, and in particular onto the inner base of a sleeve made of stainless steel, for example. In many cases, the soldering process used is what is known as SMD soldering, in which solder is first applied to a first component, followed by a second component being placed on top and soldered to the first component by heating. For the example of a resistance thermometer sensor head, a defined amount of solder in solid form ("solder wafer") is usually first inserted into the sensor head and subsequently fused to the inner base of the sensor head by heating. The sensor element is then immersed with its metallized side into the solder and thus soldered within the sensor head.Methods for producing such a solder joint for a thermometer are known, for example, from published patent application DE102006048448A1 and from the previously unpublished German patent application with the file number DE102015112199A1. In principle, the solder joint of a corresponding resistance thermometer has a significant influence on the thermal properties of the thermometer, particularly on the thermal coupling between the resistance element and the sensor head. Corresponding thermometers are manufactured and marketed by the applicant, for example, under the name Quicksens.

[0005] As an alternative to thermometers in which the resistance element is soldered to the sensor head, numerous thermometers have become known in which the temperature sensors are enclosed and / or encapsulated, in particular in ceramic powders such as magnesium oxide (MgO) or aluminum oxide (Al 2 O 3 ), or in a ceramic potting compound, in particular a hardening one. This procedure serves, on the one hand, to fix the temperature sensors. In addition, the encapsulation, for example in a ceramic powder, ensures that the connecting leads of the temperature sensor are insulated from one another and from the housing of the sensor head, in particular a sensor tip, which is often made of a metal or a metal alloy, even at high temperatures, in particular at temperatures of up to approximately 600°C. A method for producing a thermometer designed in this way is known, for example, from the published patent application DE02329239A1.To cast a temperature sensor, at least a predeterminable amount of a first and a second component of a casting compound is introduced into a mold, in particular a sensor tip. The sensor tip is then shaken and / or set into vibration at a predeterminable frequency. Such thermometers are also manufactured and marketed by the applicant and bear, for example, the name Strongsens.

[0006] To determine and / or monitor the temperature of the respective medium, the sensor element is brought into at least partial and / or temporary thermal contact with the medium by means of the immersion body. Heat exchange takes place between the device and the medium until thermal equilibrium is established. When thermal equilibrium is present, the sensor element, the immersion body and the medium have essentially the same temperature, hereinafter referred to as the equilibrium temperature. Until this equilibrium is reached, heat propagation takes place from the medium to the device or vice versa. In this context, heat propagation is understood to mean both a heat flow from the medium to the temperature sensor, corresponding to the case where the medium has a higher temperature than the thermometer, and in the opposite direction, if the thermometer has a higher temperature.The time it takes for thermal equilibrium to be established after a temperature change ΔT of the medium correlates with the response time of the thermometer. The response time fundamentally depends on the device design, the thermal conductivities of the materials used, as well as the respective geometric design and the quality of the contacts between different materials.

[0007] Now it is the case that sensor elements or measuring inserts of thermometers are often placed in an immersion body, in particular a protective tube. Such thermometer protective tubes, which can be made from ceramic or metallic materials, for example, are often used to improve the resistance of the measuring insert to chemicals, abrasion, corrosion or extreme temperatures. In process measurement technology, this is of great importance, particularly in the chemical or oil industries or in power plants, etc. However, depending on the design, fault-free function may require that the measuring insert rests on the base of the protective tube. Only then is the heat from the process quickly thermally conducted to the temperature-dependent sensor element, which is arranged in the measuring insert. In terms of design, the tip of the measuring insert is therefore pressed against the base of the protective tube by springs, for example.to ensure metal-to-metal contact. The available spring travel is typically approximately 8-10 mm.

[0008] The structure and requirements for these thermometers are determined, for example, by VDI / VDE 3511 Sheet 5 Technical Temperature Measurements and DIN 43772 Metal Protection Tubes and Neck Tubes for Thermocouples and Resistance Thermometers.

[0009] However, various errors can occur during installation or assembly of a corresponding thermometer. Due to an unsuitable design of the measuring insert to the protective tube length, unsuitable seals, or faulty installation, the spring travel may be insufficient to press the measuring insert against the base. The lack of metallic contact results in a significant reduction in measuring accuracy and an increase in the thermometer's response time to a change in the temperature of the medium. In order to ensure good contact, it is known, for example, to use so-called spring screws. These screws press the measuring insert against the base of the protective tube by screwing the measuring insert to a head housing. In this regard, reference is made to the thermometer marketed by the applicant under the name iTemp.Furthermore, documents DE102013114140A1 and DE102014101968A1 describe the use of additional spring elements to improve the contact between the protective tube and the measuring insert. JP 2015-152336 A discloses a protective tube with a removable measuring insert and a coupling unit arranged between them.

[0010] Despite the measures described, with such a design, it is not easy to verify whether the contact between the measuring element and the thermowell is sufficiently good. Another problem is the dimensioning of the thermowell and the measuring insert. It is usually unavoidable that at least a small air gap remains between the outer wall of the measuring insert and the inner wall of the thermowell, which has a detrimental effect on the measuring performance of the thermometer.

[0011] Therefore, the object of the present invention is to improve the response times of measuring devices with measuring inserts which are at least partially arranged in an immersion body.

[0012] This object is achieved by a device according to claim 1.

[0013] According to the invention, the coupling unit comprises graphite as a solid lubricant. It has become known from the prior art to arrange so-called thermally conductive pastes between the measuring insert and the immersion body. However, these have the disadvantage that the respective thermal contact between the measuring insert and the immersion body is not easily reproducible and the quality of the contact is not constant, for example due to progressive corrosion. Solid lubricants offer the advantage that they can be introduced into the immersion body more easily and, above all, precisely and reproducibly. They are particularly suitable for compensating for shape tolerances between an outer wall of the measuring insert and an inner wall of the immersion body. This allows reproducible thermal contact between the immersion body and the measuring insert.

[0014] The invention is explained in more detail with reference to the following drawings. It shows: Fig. 1 : a thermometer with a measuring insert and a protective tube according to the state of the art, Fig. 2 : a first embodiment of a device according to the invention with a coupling unit, and Fig. 3 : a second embodiment of a device according to the invention with a coupling unit, in which the coupling unit is fixed within the immersion body.

[0015] In the figures, identical features are identified by identical reference numerals.

[0016] In Fig.1 1 shows a schematic illustration of a thermometer 1 with an immersion body 2, a measuring insert 3, and electronics 4 according to the prior art. The measuring insert 3 is inserted into the immersion body 2 and comprises a sensor element 5, which in the present case is a temperature sensor in the form of a resistance element that is electrically contacted via the connecting leads 6 and connected to the electronics 4. In other embodiments, the electronics 4 can also be arranged separately from the measuring insert 3 and immersion body 2. Furthermore, the sensor element 5 does not necessarily have to be a resistance element, nor does the number of connecting leads 6 used necessarily have to be two. Rather, a different number of connecting wires 6 can be used depending on the measuring principle applied.

[0017] As already explained, the measuring accuracy, in particular the response time, of such a thermometer 1 depends to a large extent on the respective materials and, in particular, thermal contacts, especially in the area of ​​the temperature sensor 5. The temperature sensor 5 is in indirect thermal contact with the medium 7, i.e. via the immersion body 2. In order to ensure sufficient mechanical and, consequently, thermal contact between the immersion body 2 and the measuring insert 3, in particular the temperature sensor 5, a spring element [not shown] is frequently used in the prior art. Furthermore, it is customary to fill an internal volume V of the immersion body 2 with a thermally conductive paste 9, at least in a partial area 8 partially surrounding the temperature sensor 5. However, such measures cannot ensure consistent contact between the temperature sensor 5 and the immersion body 2.For example, an air gap s often remains, which can lead to a significant reduction in the response time and / or the measuring accuracy of the thermometer 1.

[0018] The present invention addresses this problem by using a coupling unit 10 that can be inserted into the immersion body 2. Numerous possible configurations for the coupling unit 10 are conceivable, some of which are described below by way of example. The precise configuration depends, among other things, on various specifications regarding the measuring device 1 and, therefore, also on the respective geometric situation.

[0019] The coupling unit 10 according to the invention is fundamentally designed to fill an internal volume V between an inner wall of the immersion body 2 and an outer wall of the measuring insert 3, at least in the partial area T in which the sensor element 5 is located, and to ensure a mechanical coupling between the immersion body 2 and the measuring insert 3. The coupling unit 10 thus serves to improve the thermal conductivity within the thermometer 1 and, consequently, the achievable measurement accuracy and the respective response time.

[0020] A possible embodiment of a thermometer 1 according to the invention is the subject of Fig. 2 The coupling unit 10 fills the internal volume V between the immersion body 2 and the measuring insert 3 in the area of ​​the sensor element 5. The coupling unit 10 thus creates a mechanical, and thus a thermal, coupling between the immersion body 2 and the measuring insert 3. Good and consistent thermal contact between the immersion body 2 and the measuring insert 3 can be ensured. According to the invention, the occurrence of the previously described air gap s is avoided.

[0021] According to the invention, the coupling unit 10 comprises graphite as a solid lubricant. According to examples that do not fall within the scope of the claimed invention, the solid lubricant contains boron nitride, copper, or brass. According to examples that do not fall within the scope of the claimed invention, various variants are conceivable. For example, the coupling unit 10 can be designed in the form of a powder, in the form of a coating, or as a solid body. Depending on the design, either the measuring insert 3 is first inserted into the immersion body 2 before the coupling unit 10 is added. Alternatively, it may be advantageous for other designs if the coupling unit 10 is first inserted into the immersion body 2 and then the measuring insert 3 is inserted.

[0022] According to the invention, the coupling unit 10 is fixed within the immersion body 2. It goes without saying that other embodiments may also include fixing the coupling unit in the region of the measuring insert 3. However, such embodiments, which also fall within the scope of the present invention, are not discussed in detail in the figures shown here.

[0023] In Fig. 3 Two possible embodiments of a device 1 according to the invention are shown by way of example, in which the coupling unit 10 is fixed within the immersion body 2. For this purpose, the device 1 comprises a fastening means 11 for fastening the coupling unit. In the embodiments shown, the fastening means 11 is part of the immersion body 2. In other embodiments not shown here, however, the fastening means 11 can also be designed as a separate unit, or as part of the coupling unit 10, or even as part of the measuring insert 3 or the immersion body 2.

[0024] According to the design according to Fig. 3a The inner wall of the immersion body 2 has a shoulder. As a result, the inner diameter d 1 of the immersion body 2 in a partial area T 1 facing away from the medium is smaller than the inner diameter d 2 in a partial area T 2 facing the medium. The shoulder ensures that the coupling unit 10 is fixed in the partial area T = T 2 . Fig. 3b shows a second possible embodiment of the fastening means 11. According to Fig. 3b The inner wall of the immersion body 2 has a projection which serves as a fastening means 11. The inner diameter of the immersion body 2 is reduced in the region of the projection compared to the inner diameter d 2 in the partial area T 2 facing the medium 7. In both cases, the fastening means prevents, in particular, an axial movement of the coupling unit 10, for example in the event that the measuring insert 3 is removed from the immersion body 2 or inserted into the immersion body 2. Bezugszeichenliste

[0025] 1Device 2Immersion body 3Measuring insert 4Electronics 5Sensor element 6Connecting wires 7Medium 8Partial area 9Thermal paste 10Coupling unit 11Fastening means sAir gap d 1 , d 2 Inner diameter of the immersion body T 1 , T 2 Partial areas of the immersion body VInternal volume

Claims

1. A device (1) for determining and / or monitoring at least one process variable of a medium (7), comprising - a measuring insert (3) with at least one sensor element (5), in particular arranged in an end section of the measuring insert (3), for recording the process variable, and - an immersed body (2) in the form of a protective tube for mounting the measuring insert (3), wherein the immersed body (2) projects at least temporarily and / or partially into the medium (7) and wherein the immersed body (2) encloses the measuring insert (3) at least in one section that faces the medium (7), in which the at least one sensor element (5) is preferably located, wherein at least one coupling unit (10) is envisaged, which can be introduced into the immersed body (2) and is configured to fill an internal volume (V) between an inner wall of the immersed body (2) and an outer wall of the measuring insert (3) at least in the section (T) in which the sensor element (5) is located, and to guarantee both a mechanical and thermal connection between the immersed body (2) and the measuring insert (3), characterized in that there is no air gap between the outer wall of the measuring insert (3) and the inner wall of the immersed body (2), wherein the coupling unit (10) has the form of a solid body and uses graphite as a solid lubricant, and wherein the coupling unit (10) is fixed in position inside the immersed body (2), and the measuring insert (3) can be removed from the immersed body (2) without removing the coupling unit (10), in particular for replacing the measuring insert (3).

2. A device (1) as claimed in claim 1, wherein the coupling unit (10) is in the form of a coating.

3. A device (1) as claimed in claim 1 or 2, wherein the coupling unit (10) is in the form of a foil or a tubular body.

4. A device (1) according to at least one of the preceding claims, wherein the coupling unit (10) is designed to dampen vibrations that occur in the immersed body (2) and / or of the measuring insert (3) by configuring the coupling unit (10) in such way that it prevents movements of the measuring insert (3) relative to the immersed body (2) in the radial direction.

5. A device (1) according to at least one of the preceding claims, wherein the coupling unit (10) is configured to facilitate a movement, in particular a reversible movement, of the measuring insert (3) relative to the immersed body (2) in the axial direction by configuring the coupling unit (10) in such way that it compensates various, in particular reversible, expansions in the longitudinal direction that occur due to changes in temperature.

6. A device (1) according to at least one of the preceding claims, wherein the coupling unit (10) and / or the immersed body (2) exhibit at least one fixing (11) for attaching the coupling unit (10) in the internal volume of the immersed body (2).

7. A device (1) as claimed in claim 6, wherein an inner wall of the immersed body (2) exhibits a ridge that serves as a fixing (11), wherein the internal diameter (d2) of the immersed body (2) in a section that faces toward the medium (T2) is greater than the internal diameter (d1) of the immersed body (2) in a section that faces away from the medium (T1).

8. A device (1) as claimed in claim 6, wherein an inner wall of the immersed body (2) exhibits a projection that serves as a fixing (11), wherein the internal diameter of the immersed body (2) is smaller in the area of the projection than in the section (T2) that faces away from the medium (7).

9. A device (1) as claimed in claim 6, wherein the fixing (11) is a face joint, in particular a mechanical joint, in particular wherein the fixing (11) exhibits at least one circlip or one retaining ring.

10. A device (1) as claimed in claim 6, wherein the fixing (11) is an adhesive.

11. A device (1) as claimed in claim 6, wherein the fixing (11) is produced at least in part from a sintered material.